Sterilizing propagation media

I don’t propagate plants from cuttings on a massive scale and most of what I do is of easy to root subjects. I have a Vitopod propagator which gives me a suitable environment for a wide range of subjects and I have been using Melcourt Sylvagrow multipurpose compost as my propagation medium.

Not everything I take cuttings of succeeds and whilst that is not unexpected, it is unwanted. Inevitably the things I am most anxious to root are the ones most likely to fail.

I use clean pots or trays to stick cuttings in and try to keep the propagator itself reasonably clean. I have a temperature/humidity sensor in the propagator and have moved the whole thing around in the glasshouse as well as provided extra shading to get the conditions within it to my satisfaction. The biggest problem was that strong direct sunlight, even on a small part of the propagator, sent the temperature rocketing and the relative humidity plunging.

Sensor data for the last month.

Sylvagrow is a peat free compost made from shredded and composted wood waste and bark. In the production process these are stacked in huge outdoor heaps to compost, a process which raises the temperature high enough to kill most plant pathogens. However, as with all such processes, it is a form of pasteurisation, not sterilisation, and it is often the case that on opening a new bag there is fungal mycelium growing in the compost. In all probability this will be a harmless saprophytic species but if a harmless species can survive, so too can a pathogenic one. Given that wood waste from forestry operations is a key ingredient and that this seems likely to be a motley mix of forestry brash which has spent some time lying on the forest floor, it seems altogether possible that it will contain organisms that are pathogens of woody plants, including the subjects I am trying to propagate.

Certain propagation media ingredients may be sterile, such as vermiculite and perlite. Peat may be close to sterile when harvested. Most other media will not be, and contamination between production and end use is not unlikely. I am not in any way singling out this particular product, it merely happens to be the one I use.

A quick online search brings up a wealth of information around the subject and I eventually settled on aiming for a heat treatment of 60°C for 30 minutes. I moistened some compost to close to field capacity and put it in a bowl in the microwave. I overshot my target temperature, in fact it went up to 90°C, from which I let it slowly cool to room temperature. There is a danger of releasing phytotoxic chemicals by overheating so I need to do some test runs to find out how long a measured volume of compost needs to get to my target temperature. Here is one of the more detailed treatments of the subject.

OK, a level one litre pot full of compost, moistened slightly, reached about 60°C with 90 seconds cooking in our 800W microwave. A further 30 seconds took the temperature to about 75°C. The temperature was very unevenly distributed initially so I stirred the compost and let it stand. If heated to just 60°C it would probably need another burst to keep the temperature high enough for long enough, whereas I think that taken to an evenly distributed 75°C then allowed to cool should see off all the baddies comfortably enough.

A couple of earlier batches, heated in the same way but without the temperature being measured, have produced encouraging results with a close to zero loss rate on cuttings stuck in the “sterilised” medium. It will become part of my propagation regime. I have Ozothamnus hookeri and Muehlenbeckia astonii to do, both have proved difficult in the past.

This morning’s batch was used for a small batch of Camellia ‘Mimosa Jury’. I’ll let you know how they do.

5 thoughts on “Sterilizing propagation media

  1. Sanitation was very important when we were in school. Many of my colleague grew crops that benefited from. I was fortunate that the crops that we grew were not so critical. We sterilized only benches, flats and items that were previously in contact with the same material. The medium was not sterile, but did not likely contain pathogens that would bother what we grew in it.


    1. It doesn’t make comfortable reading that nurseries, both of nursery stock and native species, are the most common source of environmental infections of Phytophthoras. Once plant standing areas are infected they would be almost impossible to get clean and would likely infect at least some of all subsequent crops standing on the same ground.

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      1. Phytophthora ramorum almost put us out of business. It did not come from the rooting medium though. It moved into the production area from the surrounding forest.


      2. P. ramorum would have put us out of business had we not been part of a bigger set-up. We never really knew where it came from but probably Dutch nursery stock. We like to blame the Dutch for most horticultural problems.

        Liked by 1 person

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